This invention relates to a method of mounting a semiconductor element for analyzing failures thereon.
In order to improve the yield and reliability of semiconductor elements, failure analyses and extremely important in locating defects on the semiconductor elements and in defining the causes for those defects. There have been previously porposed various approaches to analyze failures on semiconductor elements. One of such approaches is to utilize the dynamic scattering effect exhibited by nematic liquid crystals. The utilization of the dynamic scattering effect has numerous advantages including simplicity, high sensitivity, etc. and it therefore has been admiringly employed in recent years.
Methods utilizing the dynamic scattering effect of nematic liquid crystals first dispose the nematic liquid crystal in the form of a film on the main face of the particular semiconductor chip including the semiconductor elements to be analyzed, and then dispose a glass slide with a transparent, electrically conducting coating on the film-shaped nematic liquid crystal and then apply a DC voltage across the semiconductor chip and the electrically conducting coating. If the entire surface of the semiconductor chip is coated with an electrically insulating film, including a pinhole defect or defects, then that defect or each defect is operative to increase a potential of that portion of the film-shaped liquid crystal facing the same relative to the semiconductor chip and the dynamic scattering effect is developed on such a portion of the film-shaped liquid crystal. That is, external light incident upon that portion of the liquid crystal is scattered. Under these circumstances, an optical microscope can be used to locate the pinhole defect or defects.
The methods as described above have previously comprised the steps of: cutting off lengths of bonding wire connected to the semiconductor chip which is accommodated in an associated package, taking out the semiconductor chip from the package, then disposing or fixing the semiconductor chip on or to a metallic plate, and disposing an electrically insulating spacer with an opening on the semiconductor chip. The electrically insulating spacer serves to maintain the semiconductor chip electrically insulated from an electrically conducting film as will be described hereinafter while at the same time keeping a uniform, constant thickness of a film of a nematic liquid crystal subsequently dropped on the semiconductor chip through the opening on the electrically insulating film. The opening on the electrically insulating film has been somewhat smaller in dimension than the semiconductor chip. That portion of the main face of the semiconductor chip facing the opening forms an object region in which failures are analyzed.
After the formation of the film of the nematic liquid crystal on the main face of the semiconductor chip within the opening on the electrically insulating spacer, a glass slide with a transparent, electrically conducting coating has been disposed on the spacer so that the electrically conducting coating contacts the nematic liquid crystal. In this way, the semiconductor chip or an analyzed sample has been disposed after which the extent of the dynamic scattering effect which is developed in the nematic liquid crystal is examined with a DC voltage applied between the electrically conducting coating on the glass slide and the metallic plate.
Conventional methods such as described above have been disadvantageous in the following respects:
(i) Since the opening on the electrically insulating spacer is smaller in dimension than the semiconductor chip, the analysis can not be effected throughout the main face of the semiconductor chip. If the opening on the electrically insulating spacer has a dimension equal to or greater than that of the semiconductor chip then the latter is located within the opening. Therefore, it is difficult to render the thickness of the film-shaped nematic liquid crystal and maintain the electrical insulation between the electrically conducting coating and the semiconductor chip; PA1 (ii) it is difficult to perform the operation of the positioning the electrically insulating spacer on the main face of the semiconductor chip for the following reasons. The semiconductor chip is as small as a few millimeters square while the opening on the electrically insulating spacer is nearly equal in dimension to the semiconductor chip and therefore small. Also, the electrically insulating spacer is formed of a thin film such as a polyethylene film having a thickness on the order of ten .mu.m and is extremely light. As a result, if the electrically insulating spacer would have been able to be disposed on the semiconductor chip so that the opening is located on the main face of the semiconductor chip within the entire area, then the spacer might be frequently displaced from its predetermined position due to the flowing of the air and/or the nematic liquid crystal occurring after the disposition of the spacer on the semiconductor chip. In addition, the electrically insulating spacer itself is difficult to handle; PA1 (iii) when the semiconductor chip which has been separated from the associated package has been disposed on or fixed to the metallic plate, it is difficult to cause a direction normal to the main face of the semiconductor chip to coincide with the optical axis of an optical microscope through which the main face of the semiconductor chip is observed. This is because the semiconductor chip separated from the package includes the rear face having, in many cases, a brazing agent or the like fused thereto and therefore forming an even surface and not parallel to the main chip face. Accordingly, with the semiconductor chip merely disposed on or fixed to the metallic plate, the main face thereof forms, in many cases, an angle other than a right angle with the optical axis of an optical microscope involved. Under these circumstances, the use of an optical microscope with a high magnification results in the formation of an observation field having the focus not uniform over the entire area thereof. This means that the dynamic scattering effect of the liquid crystal is difficult to be detected.
Accordingly it is an object of the present invention to provide a new and improved method of analyzing failures on a semiconductor element through the utilization of the dynamic scattering effect of a nematic liquid crystal which method is free from the disadvantages of the prior art practice as described above.